In this clinical renal transplantation series, donor selection is being evaluated on the basis of serologically defined histocompatibility antigens. The mixed lymphocyte culture test is also being evaluated as a quantitative measure of histocompatibility in families, and the relationship of the stimulation index of this test between histoincompatible family members and unrelated individuals is being studied. In unrelated renal transplantation, emphasis is placed on studies of serologically defined antigens and on presensitization events. Retrospective testing with sera known to contain cytotoxins has been demonstrated to be of value in prevention of accelerated rejection phenomena, and additional technics of assay of presensitization are also under study. In particular, anti-globulin and anti-thymocyte sera enhancement of the cytotoxic test is being evaluated as a method of increasing sensitivity and accuracy of pretransplant testing. Routine renal biopsy is continuing as an important post-transplant evaluation routing. The correlation of biopsy data and long-term functional results is under way, with approximately 100 cases now completely correlated. Several unique histological observations have been made and the importance of these changes is becoming evident as a prognostic indication.